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Laying the Caffeine - Dehydration Myth to Rest
A Conversation with Dr. Ann Grandjean, Hydration Researcher and Sports Nutritionist
If you regularly enjoy a few daily cups of caffeinated coffee, tea or soft drinks, you may be relieved to learn that the Institute ofMedicine (IOM) has recently concluded that caffeinated beverages can contribute to your daily water needs. However, you mayalso wonder how this can be true, since like many people, you have probably always heard that caffeinated beverages don’t“count” toward water needs.
To understand how science challenged – and refuted – the long-held belief that caffeinated beverages are dehydrating, The Beverage Institute for Health & Wellness (BIHW) talked with noted hydration expert Ann Grandjean, EdD, FACSM, CNS.
BIHW: We’ve heard for years that caffeine is dehydrating. Now
most of the diuretic effect occurs in a relatively short period
the Institute of Medicine (IOM) says that caffeinated beverages
after ingestion, especially after consuming large amounts of
can contribute to hydration. How did this change come about?
caffeine, short-term urine collections can lead to distorted
Dr. Grandjean: Every 10 years, the IOM appoints panels of
conclusions regarding caffeine’s overall effect on daily
experts to update Dietary Reference Intakes (DRIs), which are
the official nutrient intake guidelines for the US population. The
Numerous studies gave a single large dose of caffeine,
expert panel for the 2004 DRI report on water and electrolytes
collected urine for a short period, and recorded increased
carefully reviewed the existing scientific literature related to
output.6,7,8 However, acute ingestion of caffeine at levels higher
caffeine and hydration, including a number of newer studies
than those normally consumed is known to result in short-term
published within the past decade. They determined there was
sufficient scientific evidence finding that caffeine-containingbeverages do not increase 24-hour urine volume in healthy
BIHW: Your work contributed to the change in scientific opinion
individuals compared to other beverages and that caffeinated
regarding hydration and caffeine. What did you do differently?
beverages appear to contribute to the body’s daily total water
Dr. Grandjean: If the real question is, “Does habitual
intake in amounts similar to that contributed by non-caffeinated
consumption of caffeinated beverages in amounts and
beverages.1 In other words, the common belief that caffeinated
patterns normally consumed cause dehydration?” – which is
beverages don’t contribute to hydration is a myth.
the question we were interested in – then drawing conclusionsfrom studies using caffeine-naive subjects, looking at short-
BIHW: But, wasn’t the common belief that caffeinated
term urine output, or giving amounts of caffeine higher than
beverages are dehydrating based on science?
normally consumed was not, we believed, scientifically
Dr. Grandjean: Yes, but it’s important to note that research
appropriate. Instead, we decided our clinical trials should
studies are designed to answer a specific question. For
more closely replicate the “real world” experience of caffeinated
example, much of the research conducted on caffeine was
designed to answer the question, “Does caffeine increase urine
Our clinical trial results included a counterbalanced crossover
output in caffeine-naive individuals?” This means the subjects
study involving 18 healthy adult males who consumed water
were required to abstain from caffeine for one to four days or
or water plus varying combinations of beverages, including
more before the study specifically to eliminate the impact of the
carbonated, caffeinated caloric and non-caloric colas and
body’s ability to develop a tolerance to caffeine.2,3 You see,
coffee, on four separate occasions. We found no significant
the body develops a tolerance to caffeine after about three to
differences in the effect of various combinations of beverages
five days of regular use – which greatly diminishes the weak
on the hydration status of our volunteers.9 In a second study,
diuretic effect of caffeine.2,4 As a result, although these studies
we measured the effect of two regimens – one that included
found increased urine output in caffeine-naive subjects after
drinking water as part of the dietary beverages and one that
caffeine ingestion, they also led to distorted conclusions about
did not. Again, we found no significant difference between the
the effect of caffeine-containing beverages on people who
regimens on indicators of hydration status.10 As a result, we
concluded that the popular notion that caffeinated beverages
Other design commonalities in these studies also made their
cause dehydration is a myth. Subsequently, other studies and
findings less relevant for everyday life. For instance, most
reports have confirmed our findings.11,12,13
studies collected urine for only a short time. The half-life of
Interestingly, a study from way back in 1928 involving subjects
caffeine in the bloodstream is relatively short – about six hours
who normally consumed caffeine-containing beverages
in healthy adult nonsmokers and about 3.5 hours in healthy
also showed that these beverages did not increase 24-hour
adult smokers,5 although this can vary widely. Thus, because
BIHW: You say that caffeine has a weak diuretic effect in
caffeinated beverages can lead to inadequate fluid
people who don’t regularly consume caffeine. Isn’t that
consumption. They just won’t drink as much. In some cases,
this can increase their risk of dehydration.Dr. Grandjean: As mentioned earlier, the human body develops a tolerance to caffeine after about three to five daysBIHW: If the science proves that caffeinated beverages
of regular use.2,4 This is why, even though caffeine is a weak
contribute to hydration, why do so many people still believe
diuretic, drinking caffeinated beverages is not detrimental to
the hydration status of those who regularly consume them.
Dr. Grandjean: It takes time for research that challenges
Our bodies are very good at making adjustments to maintain
long-held beliefs to become the “accepted” norm, which is
homeostasis. Since water is so critical for life, it only makes
why it is so important to help people understand the science
sense that our bodies can negate the mild diuretic effect
that disproves such beliefs. This is certainly true with the
of caffeine. Research now solidly substantiates that this
misperception that caffeinated beverages are dehydrating.
adjustment does, in fact, occur.9,11,12,13,14
So, although the science is clear, I’m afraid public confusion is likely to continue until this newer science is widely and
BIHW: But what about athletes, who have higher fluid needs?
repeatedly disseminated by educators in the classroom and
Shouldn’t they avoid caffeinated coffee, tea and soft drinks?
the media through television, magazines and newspapers.Dr. Grandjean: A critique of the published controlled research
And, of course, numerous Internet sites must be updated
shows that consuming caffeinated beverages during physical
with the correct information. Here’s the challenge: it only takes
performance does not increase urine output or detrimentally
a few lines to convey the outdated information, but much
affect performance.11 In fact, when water is consumed during
more time, work and space to accurately convey the new
the rehydration phase, a greater loss of electrolytes appears
information and put it into perspective. This interview is a
to occur than does with caffeinated beverages.11 Moreover, in
good example. It takes effort, but putting pseudo-scientific
my experience working with professional and Olympic athletes,
asking them to change their drinking habits by eliminating
Dr. Ann Grandjean is President of The Center for Human Nutrition, Adjunct Associate Professor in Internal Medicine at the Universityof Nebraska Medical Center, and a member of the University of Nebraska Graduate Faculty and the Interdepartmental NutritionProgram of the University of Nebraska-Lincoln. She also served as nutrition consultant to the University of Nebraska athletic department and the U.S. Olympic Committee.REFERENCES
Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2004). Available at: http://darwin.nap.edu/books/0309091691/html. Accessed October 11, 2006.
2. Robertson D, Wade D, Workman R, Woosley RL, Oates JA. Tolerance to the humoral and hemodynamic effects of caffeine in man. J Clin Invest. 1981;1111-1117.
3. Fisher SM, McMurray RG, Berry M, Mar MH, Forsythe WA. Influence of caffeine on exercise performance in habitual caffeine users. Int J Sports Med. 1986;7:276-280.
4. Denaro CP, Brown CR, Jacob P 3rd, and Benowitz NL. Effects of caffeine with repeated dosing. Eur J Clin Pharmacol. 1991;40:273-278.
5. Parsons WD, Neims AH. Effect of smoking on caffeine clearance. Clin Pharmacol Ther. 1978;24:40-45.
6. Passmore AP, Kondowe GB, Johnston GD. Renal and cardiovascular effects of caffeine: A dose-response study. Clin Sci (Lond.). 1987;72:749-756.
7. Neuhauser-Berthold M, Beine S, Verwied SC, Luhrmann PM. Coffee consumption and total body water homeostasis as measured by fluid balance and
bioelectrical impedance analysis. Ann Nutr Metab. 1997;41:29-36.
8. Robertson D, Frolich JC, Carr RK, Watson JT, Hollifield JW, Shand DG, Oates JA. Effects of caffeine on plasma renin activity, catecholamines and blood pressure.
9. Grandjean AC, Reimers KJ, Bannick KE, Haven MC. The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. J Am Coll Nutr.
10. Grandjean AC, Reimers KJ, Haven MC, Curtis GL. The effect on hydration of two diets, one with and one without plain water. J Am Coll Nutr. 2003;22:165-173.
11. Armstrong LE. Caffeine, body fluid-electrolyte balance, and exercise performance. Int J Sport Nutr Exerc Metab. 2002;12:189-206.
12. Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: a review. J Hum Nutr Dietet. 2003;16:411-420.
13. Armstrong LE, Pumerantz AC, Roti MW, Judelson DA, Watson G, Dias JC, Sokmen B, Casa DJ, Maresh CM, Lieberman H, Kellogg M. Fluid, electrolyte, and renal indices
of hydration during 11 days of controlled caffeine consumption. Int J Sport Nutr Exerc Metab. 2005;15:252-265.
14. Eddy NB, Downs AW. Tolerance and cross-tolerance in the human subject to the diuretic effect of caffeine, theobromine and theophylline. J Pharmacol Exp Ther.
The Beverage Institute for Health & Wellness is a scientific organization, within The Coca-Cola Company, that supports scientific research, education andoutreach to better understand the role that beverages play in nutrition and health around the world.
Log on to www.beverageinstitute.com for more beverage nutrition and hydration information.

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